1. Field of the Invention
The present invention relates to a coding apparatus and, more particularly, to a coding apparatus that achieves high-efficiency coding, which is required for transmitting or recording information signals.
2. Description of the Related Art
High-efficiency coding is an important technique for digitizing image signals. High-efficiency coding can be achieved by, for example, orthogonal transformation which transforms time-sequentially input signals into orthogonal components (for example, frequency components). Examples of such orthogonal transformation are Fourier transformation, discrete cosine transformation (hereinafter, referred to as "DCT" or "DC transformation"), and Hadamard transformation. DCT is particularly suitable for high efficiency coding of image data.
A known high-efficiency coding method employing DCT will be described. FIG. 1 is a block diagram of a high-efficiency coding apparatus employing DCT.
An input digital signal from an input terminal 1 is inputted to a blocking circuit 2, which divides the signal into DCT-unit blocks. Then, a DCT circuit 3 transforms the blocked image signal into DCT coefficients, as shown in FIG. 2, by two-dimensional DCT using 64 pixels (8 horizontal pixels.times.8 vertical pixels), as shown in FIG. 2.
Such two-dimensional DCT is generally employed for high-efficiency coding of image signals.
The thus-obtained DCT coefficients are then quantized by a quantizing circuit 4. Then, a one-dimensional array circuit (zigzag scanning circuit) 5 rearranges the two-dimensional quantized data into a one-dimensional array by scanning in a zigzag manner the 64 pixels from the DC component pixel to the pixel of the highest vertical and horizontal frequency components as shown in FIG. 2.
Then, a variable-length coding circuit 6 transforms the one-dimensional array data into variable-length codes, which are then converted at a predetermined rate by a buffer memory 7 and are output from an output terminal 7.
Such variable-length coding is a scheme which assigns a codeword of a high incidence probability with a short code length, and a codeword of a low incidence probability with a long code length.
If all of the data following a significant coefficient in a block are zero, a code EOB (end of block) is assigned for that data, thereby omitting many zero coefficients occurring toward the end of each block.
In such a one-dimensional array scheme where quantized data in each DCT block is rearranged into a one-dimensional array, the distribution of electric power is relatively localized in low-frequency component in each block, but will vary to some extent from one block to another as indicated in FIG. 3(A).
Furthermore, each block contains an EOB code of, normally, 4 to 6 bits, and such codes can be redundant components which reduce the coding efficiency.